1. Field of the Invention
This invention pertains generally to broad band antennas, and more particularly to Vivaldi or tapered slot antennas and electrical feeds thereto and bandwidth and gain extension thereof.
2. Description of Related Art
Ultra-wideband (UWB) technology is increasingly being developed for communications and other applications. Unlike narrow band systems which operate at specific frequencies, UWB transmits and receives sequences of very short (typically 50-1000 ps) pulses, i.e. pulses spread over a very broad range or bandwidth (typically several GHz) of the electromagnetic spectrum. Improved antennas are needed to facilitate rf signal transmission and reception over a very broad band range.
The Vivaldi or tapered slot antenna has been known for some time, first being discussed in a 1979 IEEE European Microwave Conference paper by P. J. Gibson, “The Vivaldi Aerial.” The antenna is described therein as “a new member of the class of aperiodic continuously scaled antenna structures, and as such, it has theoretically unlimited instantaneous bandwidth.” The common feed method of microstrip and cavity matching is shown, which greatly limits bandwidth and efficiency.
As shown in FIG. 1, the prior art Vivaldi antenna 20 is generally formed of a pair of spaced conducting plates 22, 24 on a dielectric substrate 26. The plates 22, 24 are narrowly separated at the throat 28, where the electrical feed 30 is connected, and the gap 44 between the plates expands divergingly outwards along respective curved edges 32, 34 to the respective distal tips 36, 38 of the plates 22, 24. The feed 30 is generally formed of a coaxial cable 40 connected to plates 22, 24 through an impedance matching element or circuit 42.
When signals are propagated between different electrical elements, impedance matching is an important concern. If impedance is not matched, part of the signal is reflected at the interface, and power is lost. Coaxial cables having 50 ohm impedance are typically used to bring a signal to or from an antenna. Thus, ideally, the antenna should also have a 50 ohm impedance. But the Vivaldi antenna typically has an impedance of 100 ohms. This characteristic impedance has little sensitivity to plate thickness or spacing. Therefore a matching network or cavity or other matching element or circuit must be used. Matching the antenna's balanced impedance to standard unbalanced 50 ohm feed systems is often complex and difficult.
Accordingly it is desirable to provide an improved Vivaldi antenna structure having an impedance of 50 ohms, to allow direct feed from a 50 ohm coaxial cable.